Summary:
Text columns can now display their values as Markdown-formatted text
by changing their cell format to "Markdown". A minimal subset of the
Markdown specification is currently supported.
Test Plan: Browser tests.
Reviewers: Spoffy, dsagal
Reviewed By: Spoffy, dsagal
Subscribers: dsagal, Spoffy
Differential Revision: https://phab.getgrist.com/D4326
